1. Field of the Invention
The invention relates to a quenching process for heat treated metal parts, and in particular to a system and process for quenching and cleaning such metal parts with biodegradable media.
2. Description of the Related Art
Many steel alloys are hardened and strengthened by heating and then rapidly cooling the alloy. In a typical heat treating process the alloy is heated to a temperature above the upper critical temperature (Ac3), which is dependent on the composition of the alloy, so the metal is completely in the austenite phase. The alloy is then rapidly cooled by a quenching liquid or gas (quenchant) so that it can be converted into the harder martensite phase. A sufficiently fast cooling rate is needed to minimize the formation of other phases such as bainite and pearlite which are softer than martensite and will adversely affect the physical properties required of the steel. However, the cooling rate can be controlled so that various combinations of phases can be present in the as-quenched metal.
The key to successfully accomplishing this process is the uniform removal of heat from the surface of the metal part. Continuous cooling curves showing the cooling rates for a ferrous alloy are shown in FIG. 1. The first curve (1) is designed to provide a combination of martensite and austenite in the as-quenched metal. The second curve (2) is designed to provide a fully martensitic structure in the as-quenched metal. The third curve (3) is designed to provide a combination of martensite and bainite and the fourth curve (4) is designed to provide a combination of martensite and pearlite in the as-quenched metal.
In industrial practice a number of different quenchants are used, of which the main quenchants are water, quenching oils, aqueous polymer solutions, molten salts, and high pressure inert gas.
1. Quenching in Liquid
Quenching in a liquid typically includes three stages which are illustrated in FIG. 2. These stages of liquid quenching may not occur at all points on a part at the same time.
In the first stage, vapor blanket or film boiling occurs where a thin film of vaporized liquid forms in close proximity to the surface of the metal and prevents the liquid from coming into contact with the surface to thereby cool the metal surface. This stage is characterized by a low convective heat transfer.
In a second stage nucleate boiling occurs wherein the liquid vaporizes at the surface of the metal part with a very high heat exchange. The boiling point of the quenchant determines the end of this stage.
In the third stage, convection occurs wherein the liquid is close to the metal surface and the transfer of heat occurs through convection.
A. Water Quenching
Of the four liquids used for quenching, pure water is hardly ever used because of its stable vapor phase which produces non-uniform heat extraction. The addition of one or more salts to the water speeds up the breakdown of the vapor phase, thereby increasing the quenching intensity of the water. This effect results in very rapid cooling rates at the surface of the metal components, but produces large stress gradients with a danger of cracking of the components during quenching.
B. Atmosphere Oil Quench
Quenching oils of different qualities exist with the quenching severity depending on their composition and physical properties, the most important property being the viscosity of the oil. Oil, just as water, exhibits a pronounced vapor phase followed by a nucleate boiling phase with a very rapid heat transfer in the temperature range 600° C. to 300° C. typically encountered during oil quenching.
During the nucleate boiling stage of oil quenching, extremely high instantaneous heat transfer coefficients can be achieved. This is a distinct advantage in the temperature range where pearlitic transformation occurs and one not available by gas quenching. With the breakdown of the vapor phase at the onset of boiling, however, the so-called Leidenfrost phenomenon occurs. The result is a nonuniform heat transfer rate on different surfaces of the metal parts which is dependent on a variety of variables and factors. This uneven transitory step creates large temperature differentials and is a major factor in part distortion when quenching in oil media.
C. Molten Salt
Another known quenching medium is molten salt. A molten salt bath quench does not have a vapor stage or a boiling stage. Therefore, like a gas quench, molten salt quenching provides a purely convective heat transfer with the highest heat transfer right at the start of the immersion of the components into the molten salt.
Because the salts have to be molten in order to be used, their application temperature is by nature higher than those of water and oil. They are normally used in the range about 140° C. to about 350° C. This higher application temperature has the positive effect of reducing the quenching severity in the lower temperature range where martensitic transformation takes place. This is also beneficial for uniform stress distribution which results in very low distortion of the hardened metal components.
D. Disadvantages of the Known Liquid Quenching Techniques
During the nucleate boiling stage of liquid quenching such as with water, polymer, or oil, extremely high instantaneous heat transfer coefficients can be achieved. This is a distinct advantage in the temperature range where pearlitic transformation occurs and one not possessed by gas quenching. With the breakdown of the vapor phase at the onset of boiling, however, the so-called Leidenfrost phenomenon occurs as discussed above. Moreover, petroleum-based oils and salt baths are not readily disposable because of their toxic nature. Therefore, the use of such quenching media presents environmental concerns that increase the cost of their use.
II. Gas Quenching
Forced gas quenching is a single-stage quenching of a purely convective type. Gas type, gas pressure, and gas velocity are the main control parameters. Typically, a gas quenching chamber is equipped with a powerful fan and is adapted for injecting a cooling gas at a positive pressure of up to 20 bar. The gas quenching chamber may include one or more heat-exchangers using chilled water to quickly remove heat from the quenching gas. The most common quenching gas medium is nitrogen gas. However, other gases are also used such as argon gas, helium gas, hydrogen gas, and mixtures thereof.
Quenching with high pressure gas is preferable for high hardenability alloys. Typical grades of steels for which forced gas quenching is suitable include AISI-SAE grades 8620, 5120, and 4118, 17CrNiMo6, SAE grades 9310, 3310, 8822H, 4822, and 8630. However, lower hardenability, plain carbon steels that can be carburized and oil quenched, simply cannot be hardened using a gas quench because they will not properly transform under the slower cooling rates of gas quenching. Even with high hardenability grades some consideration must be given to core hardness, because the gas quench will produce lower core hardness compared to oil quenched parts.
A major advantage of quenching under high pressure inert gas is that these same slow cooling rates translate into low distortion from quenching. Many parts that cannot be successfully oil quenched and maintain required dimensional tolerances can be High Pressure Gas Quench (HPGQ) processed and provide acceptable dimensions in the as-quenched condition.
By eliminating the non-uniform cooling of parts associated with liquid quenches that have vapor, boiling, and convective cooling all taking place simultaneously and replacing it with gas quenches that have slower cooling rates and are more uniform and purely convective, distortion can be greatly reduced because the surfaces are more uniformly cooled at slower rates. HPGQ can sometimes eliminate post-heat treatment straightening or clamp tempering operations, reduce grind stock allowances and hard machining, or replace more costly processes such as press quenching
When properly applied, gas quenching has several recognized advantages, which include safety, overall economics, reduction of secondary manufacturing operations, minimizing of dimensional variation, controllable cooling rates, part cleanliness, and overall environmental impact.
There are also disadvantages that must be considered when using HPGQ technology. These include cooling rate limitations (i.e., quench severity), reversed application of heat transfer rates (i.e., slow cooling rates in the pearlitic transformation range and high cooling rates in the martensitic transformation range), regulations and codes for the pressure vessel, and high noise levels.
III. Comparison of Quench Rates
For oil quenching, the peak of the oil cooling rate in the boiling phase is 80° C./s and takes place in the important phase of steel quenching to avoid ferrite or pearlite formation.
For gas quenching, the limited quenching speed at high temperature (pearlite transformation) and high rate at low temperature (martensite transformation).
During gas quenching, one heat transfer phenomenon is usually encountered: convection. This results in a lower heat transfer coefficient than in the case of a vaporizable liquid like oil, but in a more homogeneous cooling as all the part is approximately cooled at the same rate at a same time. It leads also to a lower distortion level of the parts.